Possible PCM2706 implementation

[jaycee]

Hi all,

I'm thinking about a DAC to connect up to my Mac Mini. One thing that occurred to me was to use the PCM2706 as a USB->I2S converter but I thought I'd try one out first. I was bored yesterday and came up with this



As you can see it's just a basic copy of the PCM2706 data sheet's sample implementation. I just wanted to make this as a basic verification of how to get the chip working. Now all I need is a sample of the chip! (TI's webpage seems to be broken at the moment though, can't log in to where i'd place sample orders)

Anyone whose familiar with this chip, anything i should change?

For reference here is the schematic

[Gcollier]

That is a pretty clean layout The only thing you might consider is running this on external power. I'm actually about to start a layout for one of these myself. I plan on using an external DAC though. I have built a couple of PCM2906 boards and been fairly happy with them, but wanted to try something a little different.

G.

[jaycee]

This is just so I can play with the chip Eventually I'll be building a DAC with optical/coax in, probably an ASRC and PCM1978 chips. I am thinking about using this chip's I2S output as an alternate input on the DAC... my Mac Mini does have optical S/PDIF out, but I thought I'd try this chip anyway.

I would obviously use an external supply in the I2S mode. A big pain is that some of the I2S signals are "trapped" by that loop of track that connects R5 and R4 to VDD. I would probably also run the thing on a decent oscillator instead of an xtal (I just happen to have a 12MHz xtal handy)

[Gcollier]

Originally posted by jaycee
A big pain is that some of the I2S signals are "trapped" by that loop of track that connects R5 and R4 to VDD. I would probably also run the thing on a decent oscillator instead of an xtal (I just happen to have a 12MHz xtal handy)

Why not just squeeze that power plane a little near R5, then add some vias to bring the I2S signals topside. I looks to me like you might even have enough room to squeeze in a header there, you could run that to another board you could stack on top for your optical/coax connections. Or just jumper the signals to the free space you have above C6.

Are you deadset on a single sided board? If you switch out some of those throughhole caps for SMD and switch to a double sided board you can get yourself a lot of space on the topside for some SMD hex invertors, external DAC etc.

I like the sound of your plans with the ASRC. Have you looked at the SRC4392 http://focus.ti.com/docs/prod/folder...t/src4392.html
I've been looking at using this and the AD1865. Unfortunately I have MUCH more reading to do before this one gets off the ground.

G.

[dddac]

if you go to I2S eventually, do not forget that the DAC output must be disabled, otherwise SPDIF data is coming out of the data pin. The datasheet is not so clear about this !

you can check my circuits for the USB to I2S converter / DAC at my
Website if you wish. It implemets the PCM2707, but that is basically the same chip ...

good luck
doede

[jaycee]

Gcollier:

The reason for not using via's etc is that I wanted to keep the board single sided as I will be making it at home. Double sided is possible but a pain to get the alignment right, and small via's are impossible. I am still thinking about getting some 10nF/100nF SMD caps though to replace the through-hole ones. I've already got 100uF 6v SMD caps as you can see (salvaged from another pcb

I chose the AD1896 as it seems much more "popular" and is also an easier looking chip to use than the SRC4392.

dddac:

I was hoping the internal DAC could be disabled as it would not be used at all Would it be possible to also leave the analog power/decoupling pins disconnected?

[dddac]

Originally posted by jaycee

dddac:

I was hoping the internal DAC could be disabled as it would not be used at all Would it be possible to also leave the analog power/decoupling pins disconnected?

If you select the I2S output, the DAC is automatically disabled !

so just what you wanted.... I am sure you can leave all pins open, but may be better to at least close of with a resistor....

doede

[jaycee]

brilliant, thanks for the suggestion... I test etched a board yesterday, TQFP32 is quite possible at home

[jaycee]

Well, it's built and it works

Pictures are here for anyone interested.

[Gcollier]

Nice Job Jaycee, glad to hear it works. Those TQFP packages are quite easy to solder at home. I don't know why people are so afraid of them (the SSOP is a bit trickier since the pins are closer together, but still not a problem).

Nice work!

G